Concomitant but Not Causal Association Between Surface Charge and Inhibition of Phagocytosis by Cryptococcal Polysaccharide

Abstract

The mechanism by which capsular polysaccharides inhibit phagocytosis is not clearly understood. We investigated the association between a negative surface charge and inhibition of phagocytosis by the capsular polysaccharide of Cryptococcus neoformans . A two-polymer aqueous-phase system containing phosphate ions was used to assess surface charge. Opsonins such as normal bovine serum and normal human immunoglobulin G reduced the surface charge on non-encapsulated cryptococci and simultaneously enhanced phagocytosis. These same opsonins had no effect on phagocytosis or surface charge of encapsulated cryptococci. F (ab′) 2 fragments of normal human immunoglobulin G neither enhanced phagocytosis nor altered the surface charge of non-encapsulated cryptococci. Addition of purified cryptococcal polysaccharide to non-encapsulated cells inhibited phagocytosis of the yeast and induced a strong negative charge at the yeast surface. Chemical modification to reduce the surface charge of either purified cryptococcal polysaccharide or intact encapsulated cryptococci produced a small loss of phagocytosis-inhibiting activity; however, all treated polysaccharide preparations retained a significant ability to inhibit phagocytosis of the yeast. These results indicated that the association between surface charge and inhibition of phagocytosis was largely circumstantial, and presence of a negative surface charge could not account for the powerful antiphagocytic action of cryptococcal polysaccharide.